In flexographic printing, also known as relief printing, ink is transferred from a pool of ink to a substrate by way of a printing plate. The surface of the plate is shaped so that the image to be printed appears in relief, in the same way that rubber stamps are cut so as to have the printed image appear in relief on the surface of the rubber. Typically, the plate is mounted on a cylinder, and the cylinder rotates at high speed such that the raised surface of the printing plate contacts a pool of ink, is slightly wetted by the ink, then exits the ink pool and contacts a substrate web, thereby transferring ink from the raised surface of the plate to the substrate to form a printed substrate.
Flexographic printing competes with other forms of printing, e.g., lithography, gravure and letterpress printing. Those involved in the flexographic printing industry are constantly striving to improve the flexographic printing process in order to more effectively compete with other printing methods. One area which has received much attention from researchers is the development of improved plates for flexographic printing.
The demands placed on flexographic printing plates are great. For instance, a flexographic printing plate must have sufficient flexibility (a mechanical property) to wrap around a printing cylinder, yet be strong enough to withstand the rigors experienced during typical printing processes. Further, the printing plate should possess a low hardness or softness to facilitate ink transfer during printing.
It is also required that the printing plate have a relief image that has a chemical resistance against the aqueous-based ink or alcohol-based ink which is usually used in flexographic printing. It is further desired that the physical and printing properties of the printing plate are stable and do not change during printing or storage.
Photopolymerizable resin compositions generally comprise an elastomeric binder, herein sometimes referred to as a prepolymer, at least one monomer and a photoinitiator. To prepare the plates, there is generally formed a photopolymerizable layer interposed between a support and one or more cover sheets that may include slip and release films to protect the photosensitive surface. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable developer removes the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing.
U.S. Pat. No. 3,850,770 discloses photopolymerizable resin compositions containing methacrylated polyether urethanes based on low molecular weight polyethylene oxides or polypropylene oxides. The patent discloses using glycols of molecular weight up to 200 Da (daltons) to produce polyurethane resins of molecular weight up to 2400 Da. Having a molecular weight higher than this is said to result in too low a hardness and too high a swell to be suitable.
U.S. Pat. No. 4,057,431 discloses photopolymerizable polyurethanes prepared with various ethylene oxide/propylene oxide copolymers. Hardness results for the samples that were provided were 95 Shore A or greater, which is generally too hard for printing on most substrates in flexography.
Methacrylate- or acrylate-terminated polyurethane oligomers diluted with various (meth)acrylate monomers and a photoinitiator is described in U.S. Pat. Nos. 4,006,024 and 3,960,572. The polyurethane oligomers of the '024 and '572 patents are derived from a diisocyanate such as toluene diisocyanate (TDI) and various mixtures polyester and polyether polyols such as polypropylene glycol adipate or polyethylene oxide/propylene oxide copolymer, and a mixture thereof The resultant printing plate can be used for printing on a wide variety of substrates, including corrugated board, various types of paper bags, and various types of cardboard packaging. However, the photopolymer resins described in the '024 and '572 patents are not stable upon storage due to degradation resulting from hydrolysis of the polyester polyol.
In order to maintain high quality, clear printing during a run it is highly desirable that a printing plate not pick up deposits of paper fibers and dried ink which would fill in reverse areas of the plate and deposit on and at the edges of the printing areas of the plate. When plates become dirty quickly during printing, the printing press must be shut down periodically during the run to clean the plates, resulting in a loss of productivity. One of the most important properties required for a printing plate for printing on corrugated boards is a high rebound or impact resilience.
It is well known in the art that polyalkylene oxide polyether polyols are polymerized through base catalysis. For example, polypropylene oxide diols are prepared by the base catalyzed propoxylation of a difunctional initiator such as propylene glycol. During base catalyzed propoxylation, a competing side reaction is the rearrangement of propylene oxide to allyl alcohol to introduce an unsaturated monofunctional species into the reactor. Due to the continual production of allyl alcohol and its subsequent propoxylation, the average functionality of the polyol mixture decreases, and the molecular weight distribution broadens, as the molecular weight of the polypropylene oxide increases. This rearrangement is discussed in "Principles of Polymerization" by G. Odian, John Wiley and Sons, .COPYRGT. 1981, pp. 515. Unsaturation is measured in accordance with ASTM method D4671-93 "Polyurethane Raw Materials: Determinations of Unsaturation of Polyols", with the result typically expressed as milliequivalents of olefin per gram of polyether (meq/gm). Typical values for unsaturation in polyether polyols vary from 0.02 meq/gm, to 0.05 meq/gm.
U.S. Pat. No. 5,185,420 discloses thermoplastic polyurethane elastomers based on polyether polyols prepared with double metal cyanide (DMC) catalysts.
U.S. Pat. No. 5,364,741 discloses using polyether diols to produce solid photosensitive resin printing plates containing chain-extended polyurethanes. Ethylene oxide/propylene oxide block copolymers of an A-B-A type structure wherein the A blocks are polyethylene oxide and the B blocks are propylene oxide with the A blocks being 10-30 weight percent of the total are disclosed as being preferred. The unsaturated polyurethanes in this reference are produced by chain extending a diol with an excess of diisocyanate, followed by further chain extension by an alkyldialkanolamine and then lastly endcapping with a hydroxyalkylmethacrylate or acrylate.